In many fields, specimen carriers in the form of support sheets, which may have a multiplicity of wells or impressed sample sites, are used for various processes where small samples are heated or thermally cycled. A particular example is the Polymerase Chain Reaction method (often referred to as PCR) for replicating DNA samples. Such samples require rapid and accurate thermal cycling, and are typically placed in a multi-well block and cycled between several selected temperatures in a pre-set repeated cycle. It is important that the temperature of the whole of the sheet or more particularly the temperature in each well be as uniform as possible.
The samples may be liquid solutions, typically between 1 micro-1 and 200 micro-1 in volume, contained within individual sample tubes or arrays of sample tubes that may be part of a monolithic plate. The temperature differentials that may be measured within a liquid sample increase with increasing rate of change of temperature and may limit the maximum rate of change of temperature that may be practically employed.
Previous methods of heating such specimen carriers have involved the use of attached heating devices or the use of indirect methods where separately heated fluids are directed into or around the carrier.
The previous methods of heating suffer from the disadvantage that heat is generated in a heater that is separate from the specimen carrier that is required to be heated. Such heating systems and methods suffer from heat losses accompanying the transfer of heat from the heater to a carrier sheet of the specimen carrier. In addition, the separation of the heater from the specimen carrier introduces a time delay or “lag” in the temperature control loop. Thus, the application of power to the heating elements does not produce an instantaneous or near instantaneous increase in the temperature of the block. The presence of a thermal gap or barrier between the heater and the block requires the heater to be hotter than the block if heat energy is to be transferred from the heater to the block. Therefore, there is a further difficulty that cessation of power application to the heater does not instantaneously stop the block from increasing in temperature.
The lag in the temperature control loop will increase as the rate of temperature change of the block is increased. This may lead to inaccuracies in temperature control and limit the practical rates of change of temperature that may be used. Inaccuracies in terms of thermal uniformity and further lag may be produced when attached heating elements are used, as the elements are attached at particular locations on the block and the heat produced by the elements must be conducted from those particular locations to the bulk of the block. For heat transfer to occur from one part of the block to another, the first part of the block must be hotter than the other. Another problem with attaching a thermal element, particularly current Peltier effect devices, is that the interface between the block and the thermal device will be subject to mechanical stresses due to differences in the thermal expansion coefficients of the materials involved. Thermal cycling will lead to cyclic stresses that will tend to compromise the reliability of the thermal element and the integrity of the thermal interface.